1. Field of the Invention
the present invention relates to a motor controller and a disk drive unit, and more particularly to a disk drive unit having accurate control of the rotational speed of a motor, such as a hard-disk drive.
2. Description of Related Art
Conventional disk drives typically use a spindle motor to rotate the disk or disks in the disk drive. The high-speed rotation (e.g., 5400 rpm) of a disk is achieved by fixing a hub, which integrally holds the disk, to the spindle motor and by transmitting the rotation of the spindle motor to the hub. A head typically flys above the disk at a predetermined fly height providing a gap between the head and the disk. When the head is positioned over the desired track, data may be read from or written to that track. When the motor does not accurately rotate the disk, it is difficult for the head to follow the track often leading to data errors when writing to or reading from the disk. Therefore, the disk drive is required to prevent these types of data errors to maintain stable high-speed rotation of the disks.
One method of controlling the rotational speed of a motor used in a disk drive is to compare a target frequency or a target phase with the frequency or the phase in synchronization with the actually measured rotational speed of the motor. The driving current of the motor is controlled by this difference. Such methods of controlling the motor speed is often referred to as the phase-locked loop (PLL) or frequency-locked loop (FLL) control methods. FIG. 1 illustrates a block diagram of a conventional FLL used for motor control.
According to FIG. 1, a target frequency and a measured frequency are provided to a frequency comparator 12 at inputs 10a and 10b, respectively. The target frequency represents the frequency of the input signal into the FLL circuit 10 and the measured frequency represents the feedback signal of the FLL circuit 10. The output of the frequency comparator 12 is coupled to a charge pump 14. The difference between the two input frequencies of the charge pump 14 control the amount of current generated and output by the charge pump 14. The output of the charge pump 14 is provided to the loop filter 16 for shaping the waveform. The loop filter 16 output is provided to a drive circuit 18. The drive circuit 18 generates and outputs a motor driving current. By controlling the driving current in this manner, a motor 20 is rotated. The rotational velocity of the motor 20 is converted to a frequency by a velocity/frequency (V/F) converter 22 and is provided to frequency comparator 12 as the measured frequency at input 10b. FIG. 2 illustrates the gain of the elements in the FLL circuit 10.
A drawback of the conventional motor speed controller using FLL or PLL control methods, such as FLL circuit 10 shown in FIG. 1, is that it is difficult to obtain a high-speed transient response. In other words, the time it takes to get the disk or disks into uniform motion is relatively slow. This results from a relatively large time constant of the loop filter in the PLL or FLL circuits. When there is an attempt to achieve high-speed and high-precision rotation of disk(s) in a disk drive, the impact of high frequency noise must be minimized. In order to filter out the impact of noise, there is the need to limit the gain, and consequently, the time constant cannot be reduced. Therefore, it is difficult to achieve a high-speed transient response with the conventional FLL or PLL circuits.
In the latest disk drives, particularly those used in personal computers, when the disk is not accessed within a predetermined amount of time, the current supply to the spindle motor is reduced or stopped in order to reduce dissipation power. Accordingly, the spindle motor is stopped or operates with reduced speed. In a power-saving disk drive such as this, if there is a request to access a disk during the time the disk is in its stopped state or being rotated at low speeds (i.e., the low power state), the rotational speed of the disk must be increased to a predetermined rotational speed. Data may be accessed from the disk once the rotation of the disk has been returned to the rotational speed at which data can be written to or read from the disk. Thus, the performance of the disk drive may be enhanced by how quickly the disk is returned to a predetermined rotational speed.